A fuel cell is a device that can produce electricity by converting chemical energy from a fuel into electrical energy through an electrochemical reaction within a fuel cell stack (hereinafter referred to as the “stack”), instead of converting the chemical energy from the fuel into heat through combustion. Fuel cells may not only provide power for industries, households, and vehicles, but may also be applied to power small electric/electronic products, especially, portable devices.
For example, proton exchange membrane fuel cells (PEMFCs), also known as polymer electrolyte membrane fuel cells, are extensively being studied as a power source for driving vehicles. Such a PEMFC includes: a membrane electrode assembly (MEA) having catalyst electrode layers, in which an electrochemical reaction occurs, attached to both sides of an electrolyte membrane through which hydrogen ions move; gas diffusion layers (GDLs) serving to uniformly distribute reactant gases and deliver electrical energy that is generated; gaskets and coupling members for maintaining air tightness of the reactant gases and a coolant and appropriate clamping pressure; and bipolar plates allowing the reactant gases and the coolant to move therethrough.
In the aforementioned fuel cell, a fuel, usually hydrogen, and an oxidizing agent, usually oxygen (air), are supplied to an anode and a cathode of the MEA, respectively, through a flow path of the bipolar plate. Hydrogen is supplied to the anode (also called “fuel electrode”, “hydrogen electrode”, or “oxidation electrode”), and oxygen (air) is supplied to the cathode (also called “air electrode”, “oxygen electrode”, or “reduction electrode”).
Hydrogen supplied to the anode is split into hydrogen ions (protons, H+) and electrons (e−) by a catalyst of the electrode layers provided on both sides of the electrolyte membrane, and only the protons selectively pass through the electrolyte membrane, which is a cation exchange membrane, to be delivered to the cathode, while the electrons are delivered to the cathode through the GDL and the bipolar plate, which are conductors.
In the cathode, the protons supplied through the electrolyte membrane and the electrons supplied through the bipolar plate meet and react with oxygen of the air supplied to the cathode by an air supply system to produce water. The movement of the protons leads to the flow of the electrons through a wire, thereby creating electric current.
A fuel cell system mounted in a vehicle is primarily made up of: a stack generating electrical energy; a fuel supply system supplying a fuel (hydrogen) to the stack; an air supply system supplying oxygen of the air to the stack as an oxidizing agent required for electrochemical reaction; and a thermal management system (TMS) removing reaction heat from the stack out of the system and controlling the operating temperature of the stack.
As is generally known, the TMS includes a TMS line through which a coolant for cooling the stack circulates, and a radiator provided on the TMS line and dissipating heat of the coolant externally. Because the reaction heat from the fuel cell system is relatively greater than heat produced by an internal combustion engine system, the radiator of the fuel cell system requires relatively high heat dissipation performance compared to a radiator in the internal combustion engine system. However, a conventional fuel cell system may not exhibit sufficient heat dissipation performance of the radiator due to constraints of installation space or other installation environment issues. Accordingly, in the conventional fuel cell system, as the time taken for the coolant to reach a predetermined temperature limit is short, the output of the stack may be lowered.
Meanwhile, as the electrolyte membrane of the MEA is fully soaked with water, the ion conductivity increases and loss caused by resistance decreases. When reactant gases of low relative humidity are continuously supplied, the electrolyte membrane may become dried out and thus longer usable. Because humidification of the supplied gases is essential in a fuel cell system, a conventional fuel cell system includes a hollow fiber membrane humidifier able to perform moisture exchange between humidified air discharged from the stack and outside air to be supplied to the stack using a hollow fiber membrane. However, such a conventional fuel cell system is problematic in that the amount of the outside air humidified by only the hollow fiber membrane humidifier may not be sufficient, and the installation of the hollow fiber membrane humidifier may be costly.